The ease with which modern computer graphic systems and high resolution copiers can generate high quality fake documents has caused a rapid and dramatic increase in document fraud. Commonly forged documents include checks, and other documents such as birth certificates, travelers checks, stock certificates, credit cards, receipts for valuables; government documents such as immigration papers, identification cards, social security cards, permits, licenses, tax documents, and other valuable documents such as property title papers, academic transcripts, diplomas, admission tickets, lottery tickets, etc. Holograms have been incorporated into security documents such as credit cards and driver licenses for many years to increase the difficulty of forging them. Holograms can be made very difficult to duplicate even for expert holographers. For examples, computer generated 2D or 3D holograms, in true color, in multiplexes, in many levels (up to 17), or holograms with covert images that are visible only under specific light or condition, etc.
Dennison discloses a method for mass-producing holograms for use with security documents (WO8903760) by coating thermoplastic onto metallized paper and/or heat-resistant plastic substrates which may be employed for embossing holograms or diffractive patterns at high speed. If non-holographic information or art work is to be printed over the top surface, the hologram can be covered with an ink-compatible layer to make it ink-receptive. However, such holograms are conventionally protected with a transparent laminate to preserve the integrity of the holograms and to deter their duplication.
A variety of patents have been issued for using non-continuous reflective holograms to deter forgery. For example, Malik U.S. Pat. No. 5,411,296 for laminating written personal data, photos, and the like between a substrate and a protected discontinuous hologram; Malik U.S. Pat. No. 5,145,212, for using analogous protected discontinuous holograms including a non-transparent structure of 2 side-by-side non-continuous holograms, each restructuring a separate image; Malik U.S. Pat. No. 5,142,383, for discontinuous metallized holograms including alpha-numeric shapes; and Malik U.S. Pat. No. 4,921,319 for surface relief holographic structure with reflection from an air interface created by adhesive or spacer and welded edges. Boswell (U.S. Pat. No. 5,464,690) for another example, describes a method of hot stamping finalized document with metallized hologram.
A magnetic layer or magnetic ink has also been used on holograms to increase the difficulty of forging. For examples, Sander (U.S. Pat. No. 4,631,222) describes a hot embossed holographic layer on top of a magnetic layer; Suess et al U.S. Pat. No. 5,383,687 disclose a method for producing an embossed holographic layer with an additional magnetic layer and a metallic layer.
With the exception of U.S. Pat. No. 5,267,753 which involves writable metallized holograms on checks, where the hologram is deliberately intended to be partially destroyed by the writing or printing, all the anticounterfeit holograms are covered with a protective top layer. In metallized holograms, however, the high reflectance from the reflective metal causes black blotches in photocopies when reproduced with a microfilm camera or photo-copier. This is an undesirable feature for the banking industry which microfilms or otherwise photocopies their checks daily. What is needed is a see-through writable hologram with lower reflectance that would permit microfilming and photocopying to yield a clearly legible copy for legitimate record purposes but clearly illegitimate when passed off as an original.
See-through holograms on paper or plastic substrate can be made by coating polymer/thermoplastic such as polypropylene, polyvinyl chloride, polyester, or styrene, etc. and embossing the hologram on it, as is well known in the industry. Slightly more reflective holograms can also be made by depositing very slight amount of metal, of the order of nanometer thickness. These polymeric layer can also accept additives to enhance counterfeit resistance while remaining essentially transparent and not too highly reflective as in the case of metallized holograms. There are several materials that are suitable as additives to the polymer used for fabricating a see-through and writeable holograms. Addition of such materials to the see-through writeable hologram can enhance their easy authentication and increase their counterfeit-resistance. Among these materials are:
(a) Thermochromic dyes that changes color upon temperature change (when pressed between two fingers, for example) can be added to the holographic polymeric resin or to the printing ink on the paper to be covered with the hologram. Borowski (U.S. Pat. No. 5,403,039), for example, describes a thermochromic printing ink for lottery tickets. In some cases, the stability of the dye can be improved by incorporating the dye in the plastic solid, or in others, by laminating it between protective layers. PA1 (b) Photochromic dyes that can reversibly change color upon exposure to light can be incorporated into the ink or into polymer comprising the hologram. Tagaya, et a, U.S. Pat. No. 5,474,715 for example, describe methods of incorporating photochromic material in thin film. PA1 (c) Fluorescent dyes with high extinction coefficients that can emit at specific wavelengths when excited with a light source at another specific wavelength. Such dyes can also be incorporated into polymeric resins used for making the hologram or printed on the document before covering it with the transparent hologram. Sachetti (U.S. Pat. No. 5,172,937), for example describes the use of fluorescent and phosphorescent dyes for attractive and protective labels. Fluorescent and phosphorescent compounds have been available from many companies such as SICPA Secureink Corporation, De la Rue Corp., etc.